Ion beam milling is widely used in the ion beam thinning units and analytical devices of structural characterization and in manufacturing technologies applying layered structures. Different ion sources have been developed according to requirements. In the field of structural research gas ion sources are used for preparation of samples for electron microscopy, e.g. ion beam thinning, for cleaning of surfaces, e.g. tunnel electron microscopy, and for investigation of buried layers in chemical analysis, e.g. Auger electron spectroscopy or secondary ion mass spectroscopy. The target is placed in a vacuum chamber the pressure of which can not exceed the value of 10.sup.-5 to 10.sup.-2. Pa in accordance with requirements of the thinning or the measurement.
Up to now, cold cathode gas ion sources have been generally used in ion beam thinning apparatus, while hot cathode ones in analytical instruments.
The simplest form of a cold cathode ion source is the cavity type dual electrode version. The advantage of such source is its compact and simple structure. However, it has some disadvantages as well. High gas pressure of 40-50 Pa should be applied inside the source to generate ion plasma and a high voltage of 2-15 keV is needed to obtain a proper ion beam. A vacuum pump with a high pumping speed of 2000-5000 l/s is necessary to achieve a low background pressure in the vacuum chamber of the thinning unit. The divergence angle of the beam is high (10-20.degree.) at the exit bore of the source due to a considerable scattering within the ion source. An additional disadvantage of that source is that most of the accelerated ions are neutralized close to the exit bore by secondary electrons induced by ion collisions. Although the high speed neutral gas beam can be used for etching samples, but it can not be deflected or shaped further by electric or magnetic field.
In Penning type gas ion sources the accelerating ions induced by ion collision from a cold cathode and travelling towards an auxiliary anode are forced to follow a spiral path. At a lower pressure even the increased mean free path is enough to result in ionizing collisions and avalanche process to generate the ion plasma. Typical values of the gas pressure in the ionization chamber of the source are in the range of 0.1-1 Pa. An extraction electrode accelerates the ions generated in the ionization chamber to the required energy and additional electrodes focus and scan the ion beam. One of the disadvantages of this type of source is its complicated construction. Since the target is at ground potential, the ionization chamber should be connected to a relatively high potential in accordance with the needed ion energy. This causes problems in the insulation, especially if cooling is required. Another disadvantage of the source is its large size which allows to place it in a fixed position only within the vacuum chamber.
The mean free path of the electrons and thus the ionization probability can also be enhanced by applying electrostatic field to force the electrons leaving the cold cathode to oscillate. These are the so called electrostatic electron-oscillating ion sources as described e.g. in EP-B1 0 267 481. This cold cathode ion source provides an ion current density with an order of magnitude higher, has a simple construction, its cooling circuit can be held at ground potential, the divergence angle of the ion beam at the exit bore is smaller than 10 even without any further focusing, while the required gas pressure within the ion source is about 0.1 Pa as in the case of the Penning type sources mentioned.
Hot cathode ion sources are primarily used in analytical instruments for surface cleaning and removing surface layers by sputtering. The ions are generated in a separate chamber. The hot cathode is situated in that chamber together with an auxiliary anode which is usually a grid. The chamber is connected to an appropriate potential determined by the ion energy. The system of further electrodes used to accelerate, shape and scan the ion beam is essentially identical with the arrangement of the Penning type sources. The value of the pressure needed inside the source is 10.sup.-3 -10.sup.-2 Pa, while the lower pressure in the vacuum chamber is ensured by differential pumping. Ion current of a few .mu.A can be gained as a maximum from said ion sources. These sources should be fixed to the vacuum chamber due to their large size.
There are gas ion sources which combine a hot cathode with magnetic field. Such duo-plasmotron type gas ion sources show favorable parameters as to the maximum current density and the needed gas pressure, but their construction is complicated, further their usage and maintenance are not easy. The sources should be fixed to the vacuum chamber due to their large size.
The most efficient sputtering can be reached by 10 keV ions. The high energy bombardment causes damaging of the target material and usually a 10-15 nm thick damaged layer is formed on the surface of the sample. This damaged layer hinders investigations of the ion beam thinned samples also in analytical spectroscopy. The thickness of the damaged layer can be decreased by lowering either the angle of beam incidence or the energy. However, the sputtering rate decreases due to the low angle and low energy. In such a case the solid state chemical reactions, e.g. carbon deposition from hydrocarbons of the residual gas, taking place on the surface of the sample can disturb the observations.
Both at ion beam thinning and at analytical investigations of buried layers it is favorable to start the etching at higher angles of beam incidence and at higher energy of bombardment which is lowered when a certain depth of etching is reached. As the position of the surface of the target is determined by the analytical arrangement, the ion source must be able to be tilted to adjust the appropriate angle of beam incidence which requires an ion source of small size. In the case of low energy ion etching, the sputtering rate is drastically decreased due to the dropped ion current generated by the lowered accelerating potential of the source.
In the case of cold cathode guns, there is a lower limit of the accelerating voltage which also determines the ion energy as 1.5-2 kV should be applied at least to gain a well collimated beam.
In the cold cathode ion source according to EP-B1 0 267 481 mentioned above there is an anode with a central bore and two symmetrical hollow space cold cathodes. Both of the hollow space cathodes have contractions to one side of the anode, while there are conical parts of the cathode at the another side protruding towards the anode. The source needs 1.5-2 kV anode potential as a minimum to generate ion beam. With the above parameters the value of the ion current is 4-6 .mu.A and the required gas pressure is about 0.5 Pa.